Apparatus for detecting the distribution of photons, e.g. of gamma ray or X-ray with no restriction as to their specific energies, transmitted or emitted through objects to study the compositions or functions of the objects are well known to the art. As an example, the technique referred to as Positron Emission Tomography (PET) can study, in vivo, biochemical functions, on the injection of tracer molecules which emit positrons in a living body. Shortly after the release of the positrons in the subject body, the positrons annihilate with surrounding electrons to produce a pair of photons, each having 511 keV of energy, and traveling in nearly opposite directions. The detection of a pair of such annihilation photons by two opposed detectors allows for the determination of the location and direction in space of a trajectory line defined by the opposite trajectories of the photons. By using a further technique, known to the art as tomographic reconstruction, to superpose the many trajectory lines obtained by surrounding the subject with an array of detectors, an image of the distribution of tracer molecules in the body can be produced.
A central feature of such apparatus is the photon detector. The latter must be capable of providing accurate estimates of the coordinates of photon detection as well as of the energy and time of arrival of each incident photon. Precise position, energy and time information is necessary to reconstruct an accurate image of the distribution of positron emitting molecules and enable such in vivo functional studies.
Several photon detectors capable of providing accurate estimates of the energy, transverse coordinates and time of photon interactions are known to the art. A notable example, of such a photon detector and apparatus for use thereof is disclosed in U.S. Pat. No. 4,750,972 to Casey et al., the disclosure of which is incorporated herein by reference and relied upon. A crystal compound termed a scintillator is generally used in fabricating detectors for the above application. A scintillator (the compound) has the unique property of emitting light upon excitation at the location of the interaction of a photon in the scintillator. A state-of-the-art photon detector takes the form of a block of scintillator compound formed into a plurality of right rectangular crystals with typical dimensions of 4.times.4.times.30 mm and arrayed at known transverse (X and Y) locations. The array is produced by either gluing side-by-side individual scintillator crystals to a support or by cutting slots in a solid piece of scintillator crystal along the direction perpendicular to a photon receiving end. Photons enter the block through the receiving end, formed by the top transverse tips of the crystals. The photons will penetrate a volume of the block over a finite distance before interacting with the scintillator. This distance varies statistically for every photon according to a known exponential distribution which depends on the type of scintillator compound used as well as the energy of the incident photons. Upon an interaction, the scintillator compound is excited and emits a quantity of light in proportion to the energy lost by the photon. The receiving end and side walls of every scintillator crystal are coated by a highly reflective compound that will trap that light in the volume of the crystal in which the interaction occurred. The crystal functions as a light guide for the emitted light, channeling the light, through multiple reflections on the coated side-walls, toward a light transmitting end of the crystal. An appropriate number of light sensing devices are optically coupled to the light transmitting ends of an array of crystals to collect the light from all crystals in the array. With proper calibration methods, the collected light allows determination of how much energy was released by the photon in interacting with the scintillator, to register the interaction in time and most importantly to locate the particular crystal which was excited by the interaction.
Since conventional detectors can only locate the interactions of photons within the total volume of a crystal, state-of-the-art detectors are said to be two-dimensional, i.e. an array of crystals provides only information as to the transverse coordinates, X and Y, of the photon interaction and leaves undetermined the position of the interaction along the longitudinal Z-axis of the excited crystal. The longitudinal coordinate can be safely omitted in imaging situations where the photon impinges at the normal angle to the receiving end of the crystal. In that case, the photon will penetrate in a direction parallel to the longitudinal axis of the crystal and interact in the volume of that crystal. Knowledge of the transverse coordinates (X and Y coordinates) of a pair of photons detected in coincidence is then sufficient to unambiguously determine the orientation and location in space of their trajectory line. In contrast, a photon which is incident along a direction that makes a significant angle with respect to the normal angle of the crystals arranged to form a detector may travel across several crystals of an array of crystals forming the detector before interacting. As it is not known through which of the crystals the photons truly entered the array of crystals forming the detector, knowledge alone of the transverse coordinates of the interactions of photons detected in coincidence is not sufficient. It leaves a significant uncertainty as to the orientation and location in space of their trajectory line.
The above-discussed uncertainty gives rise to decided difficulties. For example, the position resolution of a PET device is generally specified as the measured spatial size of the projected image of a point-like source emitting coincident photons. Misidentification of the crystal of incidence due to the deep penetration of photons impinging at an angle on the detectors formed by the array of crystals, leads to a degradation of the image resolution of conventional PET cameras that is well known to the art as the parallax error. The parallax error causes the image resolution to be significantly worsened for an emitting source of photons located at the edge of the camera than for a source located at its center. As a result, the parallax error limits the capacity of conventional cameras to image relatively large objects, e.g. a human torso. The problem hence becomes more prominent when imaging extended bodies or when attempting to bring the detectors closer to the imaged body.
It would, of course, be desirable to obviate the parallax error in PET cameras by use of detectors that can measure not only the transverse, but also the longitudinal coordinates of the photon interactions. Indeed, knowledge of the three-dimensional coordinates of the interactions of a pair of photons detected in coincidence, leaves no ambiguity as to their trajectory lines.
A recent approach in the art directed to the location of the X, Y and Z coordinates of light emissions in an array of crystals is disclosed in U.S. Pat. No. 5,349,191 to Rogers, which is incorporated herein by reference and relied upon for disclosure. The Rogers approach is that of a surface on at least a portion of the crystal walls having high light reflectivity and crystal cross-sectional dimensions such that the total fraction of the emitted light guided to the light sensing devices is substantially less for interactions occurring close to the receiving end of the crystal and substantially more for those occurring close to the light transmitting end. However, this approach leads to a continuous variation of the total collected light with the longitudinal (Z-direction) coordinate of emission. This variation must therefore be calibrated at a plurality of locations from the receiving end to the light transmitting end of the crystal. The calibration involves measurements of the collected light as a function of the known longitudinal coordinate of a collimated beam of photons impinging on a side face of every detector. While the Rogers detector provides accurate X, Y and Z positions of a point of photon-induced light emission in a pattern of a plurality of scintillating crystal light guides, that detector does require the above calibration, which is inconvenient and can be time consuming, especially with a large number of crystals.
Another approach in the prior art for providing X, Y and Z positions of photon-induced light emissions in the crystals is disclosed in U.S. Pat. No. 4,843,245 to Lecomte. That approach uses the stacking of two segmented scintillator crystals that have different scintillation decay times. As the approach relies on the availability of scintillators that have different decay times, it is practically difficult to extend this approach to more than two segments. Moreover, this would inevitably require relatively slow scintillators and likely compromise the maximum speed at which photons can be detected. Indeed, to provide a range of decay times, succeeding scintillators must be of slower speeds, and for a reasonable resolution, the last segment must be at a significantly reduced speed. As a result, the overall speed of a detector in Lecomte's approach can be no greater than the lowest speed of a segment.
U.S. Pat. No. 5,122,667 to Thompson uses a single crystal, which avoids the difficulties of the Lecomte approach, but relies on an absorbing band located at the median interaction coordinate along the longitudinal axis of the crystal. This light absorbing band divides the crystal into two regions such that a photon is equally likely to interact with the crystal in front of or behind the band. Thus, coincident events involving a pair of these crystal will divide into four equi-probable groups. The Thompson approach is said to be an improvement over the prior art approach where detector crystals are made of different segmented scintillators glued together. Thompson points out that using a segmented crystal of different materials in the segments will result in reduced efficiency if the overall crystal depth is constant, or decreased resolution and blurring, if the crystals are made deeper to retain efficiency.
Accordingly, a significant advantage in the art would follow from determining the transverse and longitudinal coordinates of photon interactions in a detector formed by crystals made of the same material. A photon detector providing this additional information requires only a straightforward calibration and allows for the correction of the parallax error affecting the current generation of PET cameras.